This invention relates to an extrusion die for forming thin-walled honeycomb structures from extrudable materials such as glass, glass-ceramics, ceramics, plastics, metals, cermets and other materials, especially those in particulate form, which are capable of being extruded through relatively small feed holes or channels whose length is several times their diameter or transverse dimension. The outlet ends of the feed holes communicate with grid forming discharge slots, these slots forming the cell walls of a honeycomb form extrudate. After the extrusion process, the honeycomb is treated to produce a rigid honeycomb structure as is known in this art.
Thin-walled honeycomb structures display utility in a variety of technologies. For example, thin-walled honeycomb structures fashioned from ceramic materials are used as catalyst carriers in catalytic converters in the exhaust system of internal combustion engines. They also are employed as radiators, catalyst carriers, filters, and heat exchangers.
The prior art is aware of a number of extrusion die constructions for forming thin-walled honeycomb structures, such as shown in U.S. Pat. Nos. 3,790,654 issued to Bagley, U.S. Pat. No. 3,824,196 issued to Benbow, U.S. Pat. No. 4,235,583 issued to Reed, and U.S. Pat. No. 4,354,820 issued to Yamamoto.
A significant drawback of certain prior art extrusion dies may be seen by reference to FIG. 1 of the Yamamoto patent and also to FIG. 1 of the patent to Reed. In both of these structures, the inlet portion of the die is provided with a plurality of cylindrical feed holes whose downstream ends terminate at the entrance or upstream portions of respective intersecting discharge slots, with alternate diagonal intersections of the discharge slots being directly fed by and aligned with the feed holes in the die.
Considering firstly Yamamoto, the material being extruded enters into the feed holes, each denoted by the numeral 2. The lower end of each of the feed holes is interrupted by tapered portions indicated by the numeral 20 in FIG. 3. Just after passing beyond the flow constricting, narrowing tapered portions 20, the extruded material flows into the outlet or .discharge end of the die, for final extrusion through the discharge slots. There is thus an abrupt change in the cross-sectional area and shape of flow at the lower ends of the generally cylindrical feed holes, such change caused by the tapered portions 20. These portions 20 may be termed land or overhanging portions.
Turning now to Reed, a somewhat similar construction is shown wherein the exit portions of cylindrical feed holes 7 are abruptly narrowed down at the entrance to the discharge slots, the latter being denoted by the numeral 9.
From a consideration of FIG. 1 of either of the Yamamoto or Reed patents, it is seen that there are four overhanging, flow constricting land portions at the outlet end of each feed hole, this overhang resulting in an abrupt decrease in cross-sectional area of the feed holes. This overhang is defined by the corners of the entrance portions to the discharge slots which are in the flow path of the extruded material. Each such overhang (being four in number for orthogonally intersecting discharge slots) is thus subject to a bending moment due to the force of the extruded material abutting or flowing against each overhang.
Such die feed hole geometry places high bending forces on these overhanging or cantilevered portions of the die. In turn, this requires either the use of die materials having greater strength, or, limits the feed hole density of dies formed from materials having the greatest strength. Further, in the event that the material being extruded contains abrasive material, such overhanging portions are subject to greater wear and hence increased die degradation than if these portions were not present. These drawbacks are present in compound and laminated dies, as well as in unitary dies.